My mechanic had just delivered my 1973 450 SL to me after having had a tune up and a new soft top replaced. We were almost out of gas so we went to put some in. He ask what kind I used and I said regular. He said I should be using super unleaded so after putting some in we started to drive away when the car started to chugged about and act like the fuel pump had a problem. He took it back to the shop and had worked on it all day to try and figure out what happened. I never had a problem before with the car using regular and I have in the past put in the higher grade. He himself is a little stunned as the car was running great up until that moment. Any thoughts on this????